In recent years, as more automobiles have been equipped with an electronic camera, the image pickup device has been required to have higher performance. In particular, it is a big theme to enlarge the brightness range of a photographic object which can be handled by the image pickup device, that is, the dynamic range (hereinafter, referred to as a D range).
Regarding enlargement of the D range, proposed is a logarithmic conversion type image pickup device (hereinafter, referred to as a log sensor) in which the output characteristic of the image pickup device has an output characteristic where the amount of the incident light is logarithmically converted into an electric signal by using the sub-threshold characteristic of an MOSFET, where a logarithmic conversion circuit having the MOSFET is added to an image pickup device made up of photoelectric conversion elements such as photodiodes arranged in a matrix (for example, refer to Unexamined Japanese Patent Application Publication No. H11-298798).
However, the log sensor is low in the apparent contrast due to its wide D range, so that its image is not very good particularly in the low brightness region. Therefore, in the linear-log sensor, there is proposed a linear logarithmic conversion type image pickup device (hereinafter, referred to as a linear-log sensor) in which a linear operation state (hereinafter, referred to as a linear characteristic) and the aforementioned logarithmic operation state (hereinafter, referred to as a log characteristic) can be switched automatically (hereinafter, referred to as linear-log characteristic), where a specific reset voltage is given to the MOSFET in order to automatically switch the two states of the log characteristic and the linear characteristic which is the original output characteristic of the image pickup device in which the incident light intensity is linearly converted into the electric signal (for example, refer to Unexamined Japanese Patent Application Publication No. 2002-77733).
Furthermore, in the linear-log sensor, there is proposed a pixel circuit constitution which cancels the variations, between the pixels, in the switching point (hereinafter, referred to as inflection point) between the linear characteristic and log characteristic for each pixel, which variations are caused by variations in the thresholds of the transistors constituting the logarithmic conversion circuit, (for example, refer to Unexamined Japanese Patent Application Publication No. 2001-36822).
Further, in the linear-log sensor, there is proposed a method in which the point (hereinafter, referred to as an inflection point) where the output characteristic (hereinafter, referred to as a photoelectric conversion characteristic) is changed from the linear characteristic to the log characteristic is detected by using light shield pixels (for example, refer to Unexamined Japanese Patent Application Publication No. 2006-140666). According to Unexamined Japanese Patent Application Publication No. 2006-140666, it is said that the position of the inflection point can be detected even if the temperature and integrating time are changed.
However, in the method disclosed in Unexamined Japanese Patent Application Publication No. 2001-36822, a problem arises that the pixel size is increased or the aperture rate of the photoelectric conversion section is lowered because of a larger number of the transistors constituting the pixel than the pixel circuit of an image pickup device having only the ordinary linear characteristic.
Further, in the invention disclosed in Unexamined Japanese Patent Application Publication No. 2006-140666, the same basic pixel circuit constitution as that of the image pickup device having only the ordinary linear characteristic is used, so that it is impossible to cancel the variations, between the pixels, in the inflection point which is caused by the variations in the thresholds of the transistors constituting the logarithmic conversion circuit.
Furthermore, in the invention disclosed in Unexamined Japanese Patent Application Publication No. 2006-140666, the reset potential connected to the reset transistor needs to be different from the potential of the power source so that the floating diffusion reset potential can be set variably, and the reset transistor needs to be formed isolated from the other transistors, and many wires are required, so that a problem arises that the pixel size is greater and the aperture ratio is smaller than the image pickup device having only the ordinary linear characteristic.